September 5, 2011

The first eukaryotic organism on Earth sported a novel invention; the nucleus. In prokaryotic cells, the nucleic material that is responsible for regulating cell function is a free-floating cyclical strand of DNA. It wasn’t until the advent of the nucleus and membrane-bound organelles that the first animal life appeared. Molecular biologists have proffered that membrane-bound organelles are the result of a faulty gene that eliminated the prokaryote’s rigid cell wall. The fluidity of the cell membrane allows for sections to flop around and fold back onto itself, forging innovations like the nucleus, the endoplasmic reticulum and the Golgi apparatus of our eukaryotic cells. Areiosan eukaryotes contain membrane-bound organelles similar to the ER and the Golgi, and while collectively these two Areiosan organelles can accomplish the same tasks as the ER and Gogli, the division of labor is wholly different from our cells.

The origin of the nucleus is thought to be accident; some have proffered that it all began with a virus. Viruses are rudimentary biological machines that aren’t even alive because they cannot reproduce on their own. Made of a strand of nucleic acid surrounded by a protein coat, a virus hijacks a cell’s ability to reproduce and infects a cell to commandeer reproduction. The origin of the virus is unclear; one hypothesis suggests that they may have evolved from plasmids, extraneous pieces of DNA get traded back and forth by bacteria in a process called lateral gene transfer. Another theory claims that viruses may trace their origins back to parasitic cells that loss much of their cellular machinery, eventually becoming a barebones reproduction machine. In general, though scientists suspect that virus may trace their origins back to a primeval time before the emergence of archaea, bacteria, and eukaryotes. Nonetheless, even though they are generally thought to be rudimentary, some viruses have been known to contain more nucleic acid within them than even some smaller cells.

The current thinking about the origin of the nucleus is that some hapless virus infected a bacterium, but through luck or some other cause, the virus did not succeed in replicating itself and taking over the cell. Or perhaps a cell unwittingly swallowed virus, engulfing it in phagocytosis. In either case, in order to reproduce, viruses have to infect a healthy cell and take over the cell’s machinery in order to reproduce. Stranded inside the cell, but unable to take the cell over or to reproduce itself, the virus has taken over control of the cell’s RNA molecules, which allow it to make proteins. But, while it has usurped the bacteria’s DNA for control of the cellular processes, the virus is stuck regulating the workings of the cell. Over generations, the virus lost genes responsible for infection and shed some of its protein coat that eased the transmission of genetic material into the rest of the cell. From here, it is easy to conceive of a proto-nucleus; the virus has control of the RNA’s inside the cell, it is tangled up in the cell membrane, it has lost the capacity to infect other cells, yet it can’t be expelled or destroyed from within the cell by any immune response. This arrangement persisted for eons until the virus resembled something like a rudimentary nucleus.

The endoplasmic reticulum of a cell is a series of membrane-bound sacs that are involved in biosynthesis of certain molecules. The ER comes in two varieties, the smooth and rough ER, which is determined by the presence of ribosomes stuck to surface. In eukaryotic cells, the ER is responsible for processes that normally take place in the plasma membrane of a prokaryotic cell, like synthesizing proteins, lipids and steroids or metabolizing carbohydrates. By no stretch of imagination, one could hypothesize that the smooth and rough forms of the ER arose from an invagination of the plasma membrane that broke off and became specialized. For instance, while a plasma membrane is capable of functions like exocytosis and endocytosis that takes in and spits out macromolecules, the ER does not have that function.

Similar to the ER, the Golgi apparatus is another membrane-bound organelle that modifies macromolecules. The Golgi complex manufactures lysosomes and tags certain molecules with a carbohydrate or phosphate marker so those molecules can be sent to a specific location within the cell or removed by exocytosis. The Golgi apparatus likely formed as the result of invagination of the cell membrane to form a tube-like organelle that could later form vesicles for packaging molecules.